The point of a velocity stack is to increase the air velocity entering the carb. I don't think smooth air matters as much (with carbs the more turbulent air you have going in the better fuel mixing you get) as how fast it is going in there and how the acoustic resonance is affecting it. This is a big difference from pods where there is usually a flat panel for the filter, and not a funnel like channel, which arrests the flow and does not help to increase intake velocity. Just so you know that this is not bunk, honda, ducati, and most other motorcycle mfgs have used this on their race bikes for 30 years, but I have never seen a factory race bike with pods.
the wrong bellmouth/ stack can actually slow intake velocity and rob you of precious hp. it is all about tuning intake length for chamber capacity. This is because a v-stack is basically a helmholtz resonator.
A Helmholtz resonator is an acoustic resonance chamber (as described by our plenum above) that modifies the acoustic frequency of a sound wave like a spring oscillating with a mass attached on the end.
you can calucuate it using the following equation:
where f = the rpm at which you get peak torque ( the natural frequency of pressure oscillations in the acoustic chamber ) , c = the speed of sound (= 340 m/sec.) , S = runner area, L = runner length, V = displacement per cylinder
A simplified version of this is using the Englemann formula for the above which also takes into account static CR of the engine:
RPM for peak torque =
642 x c x [ SQRT (S/[L x V] ) ] x [ SQRT { (CR-1)/ (CR+1) } ]
= 218,280 x [ SQRT (S/[L x V] ) ] x [ SQRT { (CR-1)/ (CR+1) } ]
I got these equations from this auto site on building v-stacked intake runners for honda car engines:
http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=471